Concrete Encasement Research Articles

Design of steel portal frame buildings for fire safety

This paper describes a study into the fire behaviour of steel portal frame buildings at elevated temperatures using the finite element programme SAFIR. The finite element analysis carried out in this report is three dimensional and covers different support conditions at the column bases, the presence of axial restraints provided by the end walls, different fire severities within the building, different levels of out-of-plane restraint to the columns and the effect of concrete encasement to the columns. From a large number of analyses, it is shown that the bases of the steel portal frames at the foundations must be designed and constructed with some level of fixity to ensure that the structure will deform in an acceptable way during fire, with no outwards collapse of the walls. The analyses also show that to avoid sidesway (i.e. collapse outwards) it is not necessary for steel portal frame columns to be fire-protected unless the designer wishes to ensure that the columns and the wall panels remain standing, during and after the fire.

Experimental limitations regarding the formation and characterization of uranium-mineral phases in concrete waste forms

Predicting the fate of low-level radioactive waste (LLW) requires understanding radionuclide–waste form interactions. Concrete encasement is one method being considered for containment of LLW. The formation of uranium-mineral phases has been investigated in simulated concrete pore fluids and waste forms. X-Ray diffraction analyses of uranium precipitates from concrete pore fluids suggest uranium salts and -silicates are solubility-limiting phases. Scanning electron microscopy-energy dispersive spectroscopic analyses of uranium-spiked concrete suggest that under conditions both under-saturated and over-saturated with respect to the formation of uranium phases, uranyl-oxyhydroxide phases precipitate within the initial two weeks. Uranyl-silicate phases form after approximately one month and uranyl-phosphate phases provide a significant contribution to uranium retention in concrete waste forms after two months. This investigation demonstrates the importance of 1) studying the interaction of uranium in the complete matrix (i.e., concrete matrix versus pore fluids) and 2) formation of uranium-mineral phases on the retention of uranium within concrete waste forms.

World heritage in odd shoes : Reconstruction of the central building of the Budapest University of Technology and Economics

The Central Building of the Budapest University of Technology and Economics is one of the largest works of eclectic architecture in Hungary, and it is a major component of the panoramic view of the Buda side, a part of the World Heritage. The imposing building of great length along the Danube was partly built on the embankment of the original riverbed. The foundations of some load-bearing walls suffered gradual settlement in the nearly one hundred years since construction. A considerable difference in settlement of foundations was produced, which endangered the serviceability and stability of the building. Construction work was to be done with minimum disturbance of the education. In 2001 and 2002 the 1 st stage of foundation reinforcement was completed. It involved stopping the foundation settlement at the southern wing by jet-grouting (jet-mix) procedure and constructing reinforced counter-slabs between longitudinal load-bearing walls. The recoverable sections of concrete foundations were encased by the shot Torkret method, and ventilation shafts were built to prevent underground sections of load-bearing walls from wetting. Undersized and cracked load-bearing walls received reinforced concrete encasement, cracked vaults and arches were also reinforced. Following the static reinforcement, the concerned building parts were renovated. The reinforcing work was designed by teachers of the University.

Steel-concrete composite coupling beams — behavior and design

Structural steel/composite beams provide a viable alternative for coupling individual reinforced concrete wall piers. Well-established guidelines for shear links in eccentrically braced steel frames form the basis of current design guidelines. However, these provisions ignore the effects of nominally reinforced concrete encasement which typically surrounds the coupling beam, and are based on overly conservative assumed deformation demands. A coordinated analytical and experimental research program at the University of Cincinnati has focused on cyclic response of steel/composite coupling beams, their connections to reinforced concrete walls, and overall behavior of composite coupled wall systems. Using the results from this study, guidelines for proper design and detailing of steel/composite coupling beams and beam-wall connections have been developed. This paper summarizes the research program, and highlights the basic concepts, important findings, and recommendations.

Analytical evaluation of the moment-rotation response of beam-to-column composite joints under static loading

The analysis of steel-concrete composite joints presents some particular aspects that increase their complexity when compared to bare steel joints. In particular, the influence of slab reinforcement and column concrete encasement clearly change the moment-rotation response of the joint. Starting from an energy approach developed in the context of steel joints, an extension to composite joints is presented in this paper that is able to provide closed-form analytical solutions. In addition, the possibility of tri-linear or non-linear component behaviour is also incorporated in the model, enabling adequate treatment of the influence of cracked concrete in tension and the softening response of the column web in compression. This methodology is validated through comparison with experimental tests carried out at the University of Coimbra.

Strength and Ductility of Concrete Encased Composite Columns

Concrete encased composite column design provisions of the American Concrete Institute Code (ACI 318), AISC-LRFD Specification, and the AISC Seismic Provisions are reviewed and evaluated based on fiber section analyses that account for the inelastic behavior of steel and concrete, including the effects of strength and confinement on the concrete's stress-strain properties. Trial column designs are analyzed to evaluate their strength and ductility as a function of the ratio of structural steel to gross column area, the nominal compression strength of concrete, and confinement of concrete by seismic hoop reinforcing. The analyses highlight known differences in the calculated nominal strength requirements between the ACI 318 and AISC-LRFD provisions and suggest a review of criteria used to establish the limits of the provisions. Compared to columns with low- to medium-strength concrete, columns with high-strength concrete (fc′ = 110 MPa) are shown to rely to a greater degree on hoop reinforcement to provide ...

Steel-Concrete Composite Columns

Structural steel shapes and tubes can be used with concrete to produce structural components. The performance of such composite systems, is better than the sum of responses of the parts taken separately. In a concrete-filled steel tube caisson, for example, the steel casing resists flexural tension efficiently while confining the concrete core as the core resists axial compression, whereas it also stabilizes the steel casing. Concrete encasement of structural steel shapes can provide lateral stability to the steel shape, restraining local buckling as well as reducing the slenderness ratio below that of the steel shape alone. Concrete encasement serves as insulation for fire and shock loading. Characteristics of the structural behavior of composite columns are described as the basis for design recommendations. Strength design principles from Load and Resistance Factor Design of the American Institute of Steel and Construction and the American Concrete Institute are used in design. Shear transfer for the effective interaction of concrete and steel is considered, and requirements for transverse bar reinforcement of shear mechanisms are discussed. Design examples of strength estimates for two types of composite columns are included.

Rehabilitation of a Concrete Tied Arch Bridge

The John Mack Bridge, constructed in 1929, crosses the Big Arkansas River in Wichita, Kansas. The bridge is generally in good condition except for the arch hangers, which exhibited extensive cracking of the concrete encasement. The cracks are all in the vertical direction and originate mostly at the top or bottom of the hangers and progressed the length of the members. The cracks vary from very fine, less than 1 mm, to cracks 3 mm wide. It is the condition of the hangers which caused concern for the structural capacity of the bridge, resulting in a call for designs to rehabilitate the structure. In order to further analyze this complex structure, a diagnostic load test was performed using a pre-weighted test vehicle at various positions on the bridge. Measurements of strains induced in both the concrete and steel were made.

Engineering Models for Numerical Analysis of Composite Bending Members∗

Abstract ABSTRACT The two-step numerical analysis of a composite beam structure is presented in this paper. The first step, based on the idea of dividing the cross section into laminas, leads to the estimation of the moment-curvature relation for different types of cross sections used in composite beams. The second step adopts this constitutive relation, which is expressed in the space of generalized stresses and strains, into finite element nonlinear code. Some numerical examples are given, to show the agreement of numerical calculations with results of the authors' experiments, when the shrinkage of a concrete encasement and stresses due to welding processes in steel beams are considered. In addition, the numerical concept presented here seems to reduce the sensitivity of the final results obtained to finite element discretization error.

Protection of Pipelines Subject to Surcharge Loads

Surcharge loads from new fills, railroads, and structures often require design of reinforcement to increase the field supporting strength of existing sewers and storm drains. Pipelines located in easement and rights of way, or designed for shallow earth cover, are most vulnerable. Because the pipe length is often short, the unit cost for reinforcement is high, often exceeding unit costs for new sewers. Load analysis, assumptions, and design details are presented. The concrete arch encasement and its load factors are reviewed. Its use and misuse are considered. References for the designer are also cited. Alternatives to increasing pipe strength are shown by examples of reinforced concrete arch and slab designs. Recommendations are made for design and construction of new pipelines and for preventing future surcharge problems. The necessity of good inspection reporting pertaining to trench width and soil description is stressed.

Fire Resistance of Protected Steel Columns

Fully developed building fires generally attain gas temperatures in the order of 2000 degrees Fahrenheit. As the mechanical properties of steel deteriorate rapidly at temperatures of about one half this magnitude, it is necessary to provide some means of keeping steel columns relatively cool during exposure to fire, with the possible exception of extremely massive steel sections. External insulation of a steel section to prevent excessive heat transfer during the expected period of fire exposure is the most common method of providing fire resistance, although internal liquid-cooling has recently proved to be a viable protection method as well. Typical forms and methods of fire protection in current use are illustrated. Light protection using low density materials applied either to the profile of a section or in box form is most popular from an economic point of view. Massive protection, particularly concrete encasement, is used in special cases and forms the subject of a separate study. External protections, which do not readily fall into either of these categories, have been labeled "complex protection" because their analysis may require special methods or engineering judgment. Box protected H-columns with core filling or very thick contour protection are examples. Liquid filling as fire protection can be accomplished by use of design methods described in Refs. 3 and 4. This paper will confine itself to the fire resistance of steel columns protected by relatively low-density materials, examining the problem by both fundamental and experimental engineering methods.

Steel Penstock Design for Tims Ford Dam

The design and construction of an embedded steel penstock is discussed. A straight line trench, from intake to powerhouse, cut into the rock under the left side of the dam reduced construction time. The concrete encasement is designed to carry the dead load of the rockfill dam and shell. Shell and stiffeners made of A 516, grade 60 are designed for internal head of 235 ft and an external head of 153 ft. Allowable working stress of 60% of yield strength gave a safety factor of three. Hydrostatic tests of the penstock in place along with 100% radiographing of all shell welds was required.

A Comparison of Concrete Encased Grounding Electrodes to Driven Ground Rods

An experimental study was made to compare the efficacy of concrete encased grounding electrodes to that of driven ground rods for grounding residential and small commercial electric services. This study extended over 14 months and included a very wet winter and a dry summer. The ground resistances of both grounding systems were monitored for the first 5 months. Current from an isolated 120/240-volt system was then circulated through both grounding systems for the following 6 months. At the end of the 14-month period, fault current from power systems of 2.4 to 20 kV were applied to both grounding systems. The ground resistance variations were recorded in all cases.

Experiments on the strength of reinforced and prestressed concrete beams and of concrete-encased steel joists in combined bending and torsion

Summary A series of reinforced and prestressed concrete beams and concrete-encased steel joists was tested in combined bending and torsion, with ratios of bending moment to twisting moment varying from pure bending to pure torsion. In all, 33 full-size beams were tested. Measurements of defiexions and strains due to bending and to torsion were made at selected points. The agreement between theory and experiment was generally good. The torsional strength of concrete can be augmented by suitable shear reinforcement. The resistance of the reinforced concrete beams to torsion, however, still depends on the tensile strength of the concrete; it is not therefore possible to gain an increase in strength comparable with that resulting from the addition of tension reinforcement to a plain concrete beam subject to bending. On the other hand the insertion of even a small quamity of shear reinforcement turns the abrupt torsion failure of plain concrete into a gradualfailure accompanied by considerable cracking and deformation. The torsional strength of a concrete-encased steel beam is approximately the same as that of a similar plain concrete beam. The “1” shape is fundamentally a weak one in torsion and the concrete encasement therefore results in a considerable increase in the torsional strength of the steel joist, provided that the joist is surrounded with a nominal amount of stirrup reinforcement or expanded metal to ensure that the steel and concrete act together. Very large increases in torsional strength can be obtained by prestressing, since the initial prestress in compression must be overcome before the concrete can be stressed in tension. The prestressing force, however, increases the rate at which cracks propagate during failure, which therefore tends to assume a violent character. Two distinct types of failure can be distinguished in beams subject to combined bending and torsion. A primary torsion failure occurs with a cleavage fracture normal to the direction of the principal tension. It is governed by the criterion of a constant maximum tensile stress. In bending, tension failure is prevented by the steel, and a primary bending failure therefore results in the crushing of the concrete on the compression face. In reinforced concrete and concrete-encased steel beams the addition of bending increases the resistance to torsion. The paper concludes with detailed recommendations for the design of reinforced and prestressed concrete beams and concrete-encased rolled steel joists in torsion, and in combined bending and torsion.

The encasement of rolled steel joists in concrete

Summary: The existing allowances Jor concrete encasement of steel joists are shown to be not as comprehensive as they might be. The testing of specimens which have been chosen to give as wide a picture as possible of the general behaviour of the unit is described, together with the results, observations, and conclusions derived therefrom. Attention is paid to relations between the maximum steel and concrete stresses, deflexions, increases in strength, bond between the members and the stability of the compression flange. An elastic theory as adapted for the augmented steel is recorded.